A lot of people get very upset about doping in sport. They may well have a point—but maybe a little scientific perspective is required. Randal Monroe puts the issue well in the alt text of today's XKCD cartoon:

"A human is a system for converting dust billions of years ago into dust billions of years from now via a roundabout process which involves checking email a lot."

We are a all just complex assemblies of chemicals which ingests and expels other chemicals. It's not necessarily clear why some of those chemicals are unacceptable. Discuss. [XKCD]